1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser.
2. Description of the Related Art
As a configuration of a surface emitting laser, a vertical cavity surface emitting laser is known in which the active region thereof is put between two reflective mirrors arranged on both sides thereof to form a resonator in the direction vertical to the substrate surface thereof and to emit a light into the vertical direction from the substrate surface.
Because the vertical cavity surface emitting laser has the following many technological advantages, the vertical cavity surface emitting laser has been actively researched.
That is, this surface emitting laser operates with a low threshold and low power consumption and is also arranged to emit a circular spot light so that the laser can be easily coupled with an optical element to form an array therewith.
However, on the other hand, it is difficult for the surface emitting laser to obtain a gain necessary for oscillation because the active region thereof is small.
Accordingly, high reflectivity equal to or more than 99% is required for a pair of distributed Bragg reflector (hereinafter referred to as a DBR mirror) constituting the resonator.
In order to realize the high reflectivity, several tens of stacked layers are needed in the case of a semiconductor mirror.
Due to the layer thickness of the stacked layers, the surface emitting laser has the problems of heat easily filled in the resonator, the large threshold, and the increasing electric resistance thereof which makes current injection therein difficult.
As a resonator mirror that can be replaced with such a DBR, a first non-patent document (V. Lousse et al.: Opt. Express 12 (2004) 1575) reports the wavelength dependency of a reflected light and a transmitted light in the case of using a slab-type two dimensional photonic crystal as a mirror.
The photonic crystal is a structure in which a refractive index variation of the order of a wavelength of light is artificially formed, that is, a periodic refractive index structure in which media having different refractive indices from one another are periodically arranged.
The technique disclosed in the first non-patent document periodically forms holes in a material having a high refractive index to form an air hole (hole) type two dimensional photonic crystal as a two dimensional photonic crystal.
Then, it is reported that the light having a predetermined frequency is reflected from a plane of the two dimensional photonic crystal at the efficiency of almost 100% if the light is made to enter the plane from the direction almost vertical thereto.
A reflective mirror of the vertical cavity surface emitting laser can be formed of a very thin film by using such a two dimensional (or one dimensional) photonic crystal as the reflective mirror in a vertical arrangement to the resonance direction of a light.
That is, the reflective mirror, which has been conventionally formed of a thick multilayer film of the order of about several μm, can be formed of a very thin film of the order from several tens nm to several hundreds nm.
Consequently, the problems such as the difficulty of heat radiation and the electric resistance caused by the layer thickness of the reflective mirror can be decreased.
In the following, such a reflective mirror will be referred to as a photonic crystal mirror.
A second non-patent document (H. T. Hattori et al.: Opt. Express 11 (2003) 1799) discloses a numerical calculation example of a surface emitting laser structure that configures a resonator by combining the one dimensional photonic crystal mirror with a DBR mirror as an actual surface emitting laser device.
To put it concretely, as illustrated in FIG. 2, the calculation was performed assuming that the layers (cladding layers) above and below the layer (core layer) formed in a periodic refractive index structure were air layers.
A region 206 on the lower side in FIG. 2 is called as an air gap layer. In FIG. 2, the surface emitting laser structure includes a semiconductor substrate 200, a DBR mirror 202, an active layer 204, the air gap layer (cladding layer) 206, a photonic crystal mirror (core layer) 208, and holes 210.
However, in the configuration of the element of the second non-patent document shown in FIG. 2 thereof, because the air gap layer 206 is formed directly under the photonic crystal mirror 208, it is difficult to inject carriers into the active region 204 arranged directly under the photonic crystal mirror 208 when the element is driven by current injection.